1. Technical Field
The present invention relates to an ink set having a wide color reproduction range (gamut), in particular, an ink set having a wide color reproduction range of a blue region in a dark portion.
2. Related Art
As a color model, a CIE/L*a*b* color system is known. According to the CIE/L*a*b* color system, any color (spectrum) in a visible light region that can be observed by human beings is able to be expressed by chromaticity coordinates.
In three parameters (L*, a*, and b*) of the CIE/L*a*b* color system, L* denotes luminance (brightness) of a color, and a* and b* denote chromaticity representing hue and saturation, respectively. Here, L*=0 means black, and L*=100 means white; a* means the position of a color between red and green, wherein a negative value represents green, and a positive value represents red; and b* means the position of a color between yellow and blue, wherein a negative value represents blue, and a positive value represents yellow. Since the L*a*b* color model is three-dimensional, a color space is expressed by a three-dimensional space, wherein the luminance is shown on the vertical axis.
However, the L*a*b* color model is only an index for conceptually explaining a color space, and the color reproduction range thereof has limitation. That is, for example, in the case that an image is formed on a recording medium using a pigment-based ink or ink set, there is a color that cannot be reproduced by the colors of the L*a*b* color model.
At the same time, ink compositions containing various pigments having high chromogenicity have been proposed. JP-A-2006-282810 discloses that an ink having high chromogenicity and being excellent in ink reliability such as ejection stability, storage stability, and clogging can be obtained by an ink composition containing a quinacridone solid solution pigment, a polyether-modified polydimethylsiloxane compound, and water. JP-A-2007-186641 discloses that an ink excellent in ink reliability such as printing quality, ejection stability, storage stability, and clogging can be realized by an ink composition containing a water-insoluble vinyl polymer, water, and a surface-treated pigment (yellow, magenta, or cyan) that is composed of a solid solution pigment formed of specific two or more pigments and has a hydrophilic dispersing group bonded to the surface thereof.
Incidentally, the color reproduction of a blue region in a dark portion, particularly, a region where the L* value is 20 or less, has limitation. That is, since it is difficult to exhibit chromaticity and saturation when the luminance is low, a color having an L* value of 20 or less and an a* value of 30 or more or a b* value of −75 or less in the CIE/L*a*b* color system, for example, a color of (L*, a*, b*)=(19.4, 30.0, −72.6) or (L*, a*, b*)=(19.4, 37.6, −76.8), cannot be reproduced on an image recording medium with liquid droplets (ink).